The invention relates to a process for the efficient production of high-purity bisphenols.
Bisphenols are important raw materials for the production of polymers such as epoxy resins or, in particular, polycarbonates. High standards of purity are required of the bisphenols used for this purpose, so that for economic, large-scale production processes, in addition to the attainment of high conversion rates and selectivities in the reaction, the working-up steps are also of particular importance.
In the principally known processes, bisphenols are produced by condensing carbonyl compounds with aromatic alcohols in the presence of acid catalysts. A process for the industrially important production of 2,2-bis(4-hydroxyphenyl)propane (BPA) is the reaction of acetone and phenol in the presence of cross-linked sulfonated polystyrene resins (ion-exchange resins). Here a phenol/acetone ratio of at least 5:1 is established. To attain higher selectivities, cocatalysts are used; these are either homogeneously dissolved in the reactants or fixed to the ion-exchange resin by covalent or ionic bonds.
The object is to separate off the secondary products formed in the above process for producing BPA from the BPA by suitable procedures and to completely remove excess phenol from the product. In addition, the side streams formed during these procedures should be economically recycled in the overall process.
A mean of achieving these objects which has been described in the literature is the isolation of BPA-phenol adduct crystals from the reaction solution by suspension cystallisation, with or without prior distillation for the removal of water, acetone and phenol (EP-A 829 464, EP-A 522 700 and EP-A 671 377). Here, the highly phenolic mother liquor obtained during the filtration of the adduct crystals, optionally after the insertion of a rearrangement reaction, is returned to the front of the reaction unit and replenished with fresh phenol and acetone. In order to achieve higher purities, the suspension crystallisation may optionally be carried out repeatedly in series. A disadvantage of this process is the necessity of using expensive apparatus for the crystallisation and for the solid-liquid separation. Moreover, the coating of the surfaces of this apparatus with BPA or BPA-phenol adducts presents a problem during the suspension crystallisation and necessitates a regular cleaning of the surfaces by melting the deposits (EP-A 718 267).
In order to, circumvent these problems, processes which dispense with an adduct crystallisation and consequently with the production of a circulating flow of mother liquor have been proposed in EP-A 758 637. Here BPA is carried out by working up the flow of reactants in a cascade of purification steps involving distillation, the flow of reactants being purified without the generation of return flows of acetone, water, phenol and secondary products. Disadvantages of this process are the heat stress on the product owing to the high temperatures during the distillation, the high energy costs of the distillation cascade and the high loss of raw materials as a result of dispensing with rearrangement and return.
EP-A 785 181 describes the working-up of the reaction solution, likewise dispensing with an adduct crystallisation, by a combination of vacuum distillation for the removal of acetone, water, phenol and possibly secondary components and subsequent melt crystallisation. In this procedure as well, there are inevitable losses of raw materials as a result of dispensing with a return of the flows of BPA-containing secondary products formed during the purification.
The process according to the invention for the working-up of high-purity BPA avoids the disadvantages described above by carrying out the adduct crystallisation from the flow of reactants in the form of a layer crystallisation.